1. Field of the Invention
The invention relates to an apparatus for classifying particles and in particular to an apparatus for separating particles smaller than a given size from a mixture of particles and air.
2. Description of the Related Art
A classifier of the type described herein is an air classifier with a rotating classifier wheel similar to that shown in U.S. Pat. No. 4,528,091. The carrier air with particles entrained therein passes through blades of the classifier wheel to the inside of the wheel. The air flows from the outside of the wheel to the inside in a counter-centrifugal direction. The blades are positioned in a crown arrangement with their elongated dimension extending parallel to the axis of rotation of the wheel. The blades are fixed with their first ends inserted in recesses in a circular disc mounted on the classifier wheel hub. The second end of the blades are mounted in an annular shroud disc.
The classifier usually has a housing with a vertical axis. The housing has a cylindrical upper portion which contains the classifier wheel or wheels. The housing has a funnel shaped lower portion to direct the oversized particles downward.
The material to be classified is fed into the classifier together with the carrying air, as shown in British patent No. 927,876. Alternatively, the material and carrying air may be introduced into the classifier separately as shown in German patent No. 17 57 582. In either version the coarse material is withdrawn through a bottom opening in the lower portion of the housing, and the finer material is entrained in the carrying air and travels through the classifier wheel and out an outlet stack emerging from the classifier near its top.
Air classifiers of this type are compact and simple to operate. Even when used with material that is difficult to classify, selective classification can be achieved down to a range below 10 .mu.m in grain size while maintaining the fine material free from, oversized granules. The separation limit which can be achieved depends on the circumferential speed of the classifier wheel. The smaller the desired separation limit is, the higher the rotational speed of the classifier wheel must be. However, as the rotational speed of the classifier wheel increases, the wear on the component parts of the classifier wheel also rapidly increases. The increased wear through abrasion causes an increased level of impurities in the finished product from the abraded pieces of the wheel. In relation to normal quantities of product throughput, such impurities are relatively slight and can generally be tolerated. This is not the case when high purity or highly abrasive material is handled. In the case of high purity materials, such as fluorescent substances, ceramic materials, materials for dental prosthesis and the like, even a small amount of impurities renders the product unusable. In the case of abrasive materials, such as those having a Mohs hardness of four or more, the rapidly moving components of the classifier wheel, and particularly the blades, are subject to intense wear. Economical operation of the classifier is therefore not possible when classification of such material is necessary.
It is known to coat surfaces subject to the danger of wear with a wear-resistant ceramic material, for example, in spiral fluid-energy mills. Coated parts, however, cannot be used where they will be subjected to tensile or bending stresses. Such stresses will cause material cracking due to the difference in modulus of elasticity between the support material and the coating material. Parts coated with ceramic material are consequently not suitable for use as classifier wheel blades.